Weird behavior when writing and reading file

Question

When executing the following code, after the second read, file gets filled with zero until being 4096 bytes large. I can't figure out why :

f = open("file.txt", "w+")
f.read()
# gives ''
f.write("Hello")
f.read()
# filled with \x00,\x00
f.close()
# file is 4096 bytes large...

Fabian : I do it with my OS Blender : same result with an empty file and r+ (I use w+ in the example to create the file if not present) — Lapin-Blanc, Oct 26 '12 at 08:19
I didn't get the result on my machine, what's your platform? — Marcus, Oct 26 '12 at 08:24
I think your can't `read()` after `write()` without a [`flush()`](http://docs.python.org/library/stdtypes.html#file.flush) in between. — Paolo Moretti, Oct 26 '12 at 08:25
Which OS? Same as Marcus, I can't reproduce this on Linux, x86-64. — , Oct 26 '12 at 08:29
Paolo, it's true that if i'm doing this, i don't get this behavior anymore... Still weird though, any pointer on the fact that you can't read after write ? — Lapin-Blanc, Oct 26 '12 at 08:31

score 3 · Accepted Answer · answered Oct 26 '12 at 08:30

3

Best way to solve your problem: don't mix read() and write().

Otherwise: after the write(), use seek() before the second read() to read your file from the beginning:

f = open("file.txt", "w+")
print f.read()      # prints ''
f.write("Hello")
f.seek(0)
print f.read()      # print 'Hello'
f.close()

answered Oct 26 '12 at 08:30

sloth

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@PaoloMoretti True. Also note that the behaviour of the OPs code depends on the OS/filesystem you use. While the code would probably work on Linux with Ext3/Ext4, using it on Win7 with NTFS will result in a bunch of garbage written to the file. – sloth Oct 26 '12 at 08:40

score 2 · Answer 2 · answered Oct 26 '12 at 08:36

This is probably related to the filesystem and/or partition. I'm not fully up to speed on the details, but it is likely that 4096 bytes (i.e. exactly 4Kb) is the minimum size of a file in your filesystem and partition. AFAIK, the partition has a block size (often 4096 bytes), and files can only be allocated to whole blocks. So a file is always a multiple of 4096 bytes. I think that some filesystems have a way around this (e.g. reiserfs) by combining small files.

Weird behavior when writing and reading file

2 Answers2

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